1. Field Of The Invention
The invention relates generally to hydraulic systems for lifting loads, more specifically to such systems wherein lifting speed may be varied in response to changes in the magnitude of the load, and most specifically to the adaptation of such systems for use in industrial lift trucks.
2. Description Of The Prior Art
Hydraulic circuits for lifting heavy loads at low speeds and light loads at high speeds are known in the prior art. The prior art circuits have suffered from certain disadvantages, however. Most have been suitable for use only in lifting systems employing a single lift cylinder. See, e.g., U.S. Pat. No. 3,071,926 to Olson et al. Those which have been adapted for use in multiple cylinder systems have disadvantageously required manual selection of the speed of operation and/or have failed to effectively provide for smooth, efficient operation. The latter disadvantage is due to the fact that dual speed operation is effected in these systems by disenabling certain cylinder working areas during high speed operation, but provision is not made for maintaining fluid in these areas while minimizing system pressure losses and component complexity. Example of such systems may be seen in U.S. Pat. Nos. 3,530,767 and 3,824,896 to Shook and Tull III.